1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a transflective liquid crystal display having an improved response time as well as a wide viewing angle.
2. Description of the Related Art
Generally, liquid crystal displays of active matrix driving scheme display a natural-like motion picture in use of a thin film transistor (hereinafter, referred to as TFT) as a switching device. The liquid crystal display (hereinafter, referred to as LCD) can be made smaller than a cathode ray tube CRT. Thus, an LCD can used in a portable television or in a monitor for a lap-top computer.
The liquid crystal display needs a separate light source because it is not self-luminous device. The liquid crystal display can be classified generally into a transmissive type or a reflective type device. The transmissive liquid crystal display has a backlight unit installed to face a rear substrate of two transparent substrates between which liquid crystals are injected, and incident light from the backlight is transmitted through the liquid crystal display to the display surface. The reflective liquid crystal display has a special surface formed on the rear substrate of two transparent substrates between which liquid crystals are injected, and reflects an external light incident upon the rear substrate that came through a display surface from an external light source, such as a separate auxiliary light. Recently, a transflective liquid crystal display has been suggested that uses both a backlight and external light incident upon the rear substrate that came through a display surface from an external light source.
FIG. 1 is a sectional diagram representing a transflective liquid crystal display of the related art. As shown in FIG. 1, the transflective liquid crystal display of the related art includes liquid crystals 20 injected between an upper plate 10 and a lower plate 30, upper/lower compensation films 14 and 42, an analyzer 16 installed on the upper compensation film 14, a polarizer 40 installed under the lower compensation film 42, and a backlight unit 50 located under the polarizer 40. The upper plate 10 also includes a dual color filter 8 (hereinafter, referred to as DCF), a common electrode 4 and an upper alignment film 6 that are sequentially formed on the upper substrate 12.
The DCF 8 is formed to be different in thickness at areas corresponding to a reflective part R and a transmissive part T of the transflective liquid crystal display. The light incident to the reflective part R passes through a color filter twice, and the light incident to the transmissive part T passes through the color filter once. Thus, the DCF 8 of the transmissive part T is made thicker than the DCF 8 of the reflective part R in order to reduce a color difference at the reflective part R and the transmissive part T. Accordingly, the DCF 8 of the transmissive T is twice as thick as the DCF 8 of the reflective part R.
The upper plate 10 further includes a common electrode 4 and an upper alignment film 6 that are sequentially formed on the upper substrate 12. A common voltage Vcom is applied to the common electrode 4 to control the movement of the liquid crystals 20. The upper alignment film 6 is completed by performing a rubbing process after spreading an alignment material such as polyimide on a common electrode 4.
FIG. 2 is a diagram representing a lower plate illustrated in FIG. 1 in detail. The lower plate 30 includes a TFT, a pixel electrode 34, a protective layer 36, a reflective plate 38 and a lower alignment film 44 formed on the lower substrate 32. The TFT includes a gate electrode connected to a gate line GL, a source electrode connected to the data line DL and a drain electrode connected to a pixel electrode 34 through a contact hole. Further, the TFT includes a gate insulating film to insulate the gate electrode, source and the drain electrodes and semiconductor layers to form a conductive channel between the source electrode and the drain electrode when a gate voltage is supplied to the gate electrode. The TFT selectively supplies a data signal from the data line to the pixel electrode 34 in response to a gate signal from the gate line. A voltage difference between data signal from the TFT and the common voltage Vcom from the common electrode 4 causes liquid crystals to rotate, and light transmittance is determined in accordance with the extent of rotation of the liquid crystals.
As shown in FIG. 2, the pixel electrode 34 is located at a cell area divided by the data line and the gate line and is made from a transparent conductive material with high light transmittance. The pixel electrode 34 is formed on the protective layer 36 spread on the entire surface of the lower substrate 32, and is electrically connected to the drain electrode through the contact hole formed on the protective layer 36. The lower alignment film 44 is formed by way of performing a rubbing process after spreading an alignment material on an upper part of the lower substrate 32 provided with the pixel electrode 34.
As shown in FIG. 1, the reflective plate 38 is formed at an area corresponding to the reflective part R to reflect incident light from the outside. The reflective plate 38 is made from AlNd metal. The backlight unit 50 is a light source that generates light to transmit the generated light toward the display surface corresponding to the transmissive part T. The external light incident to the reflective part R is reflected at the reflective plate 38 through the liquid crystal layer 20 and is radiated to outside through the liquid crystal layer 20. On the contrary, a visible ray generated at the backlight unit 50 installed within the liquid crystal display other than the external light is transmitted through the liquid crystal layer 20 to progress toward the display surface. Due to such a characteristic, a cell gap is made different in order to make an optical phase difference the same at the reflective part R and the transmissive part T. In other words, a cell gap d2 of the transmissive part T is made twice as wide as a cell gap d1 of the reflective part R, thereby making the optical phase difference the same.
The liquid crystals 20 injected between the upper plate 10 and the lower plate 30 are in an Electrical Controlled Birefringence ECB mode where the liquid crystal cells are aligned parallel to each of the alignment films 6 and 44 of the upper plate 10 and the lower plate 30, and move to be parallel to the direction of electric field when applying the electric field. If the external light is sufficient enough, the external light incident to the upper plate 10 is reflected by the reflective plate 38 of the reflective part R to progress toward the front surface, thereby implementing a picture. In addition or in the alternative, if the external light is not sufficient enough, the light generated from the backlight unit 50 is controlled by the arrangement of the liquid crystals 20 of the transmissive part T, thereby implementing a picture.
The transflective liquid crystal display has an advantage in that it can be used without depending on external lighting conditions, such as day or night, because a picture can be implemented using the external light and/or the backlight unit 50. However, a need arises to reduce the response time of liquid crystals such that the viewing angle can be made wider.